In general, a solar cell is a device that converts light energy of the sun into electrical energy. The solar cell is a device that produces electricity using sunlight which is an infinite energy source, a representative solar cell is a silicon solar cell, which is already widely used in daily life, and recently, studies have been conducted on a dye-sensitized solar cell as a next-generation solar cell. The dye-sensitized solar cell is a photo electrochemical solar cell which has higher efficiency and much lower manufacturing cost per unit than the existing silicon solar cell and thus has a possibility of replacing the existing silicon solar cell.
The dye-sensitized solar cell was representatively reported by a research team of Michael Gratzel, et al., at the Swiss National Higher Institute of Technology in Lausanne (Ecole Polytechnique Federale de Lausanne, EPFL) in 1991 (see U.S. Pat. No. 5,350,644 “Photovoltaic cells”). In the structural aspect, one of two electrodes of the dye-sensitized solar cell is a photoelectrode including a transparent conductive substrate on which a semiconductor layer, on which a photosensitive dye is adsorbed, is formed, and a space between the two electrodes is filled with an electrolyte.
Below, the operating principle of the dye-sensitized solar cell is based on when solar energy is absorbed into a photosensitive dye adsorbed on a semiconductor layer of an electrode to generate photoelectrons, the photoelectrons are conducted through the semiconductor layer, and thus are transferred to a conductive transparent substrate in which a transparent electrode is formed, and the dye, which has lost electrons and thus is oxidized, is reduced by an oxidation⋅reduction pair included in the electrolyte. Meanwhile, the electrons, which reach a counter electrode, which is an opposite electrode, through an external electric wire, again reduce the oxidation⋅reduction pair of the oxidized electrolyte to complete the operation process of the solar cell.
Meanwhile, the dye-sensitized solar cell includes various interfaces such as an interface between a semiconductor and a dye, an interface between a semiconductor and an electrolyte, an interface between a semiconductor and a transparent electrode, and an interface between an electrolyte and a counter electrode, as compared to the existing solar cells, and it is an implemental key to the dye-sensitized solar cell technology to understand and control the physical⋅chemical actions at each interface. Further, the energy conversion efficiency of the dye-sensitized solar cell is proportional to the amount of photoelectrons produced by the solar energy absorption, and in order to produce a large amount of photoelectrons, it is required to manufacture a photoelectrode including a structure which may increase the amount of dye molecules adsorbed.
Meanwhile, an electrolyte used for the dye-sensitized solar cell may be classified into a liquid electrolyte, a gel-type electrolyte, and a solid electrolyte depending on the properties thereof. When a solar cell is manufactured by using a liquid electrolyte, there is an advantage in that the energy conversion efficiency is increased, whereas there is a disadvantage in that a solvent included in the liquid electrolyte may leak or be volatilized depending on an increase in external temperature or the sealing state of the solar cell, thereby reducing the lifetime of the solar cell. In contrast, when a solar cell is manufactured by using a solid electrolyte, leakage or volatilization problems of the electrolyte do not occur, but there is a disadvantage in that the energy conversion efficiency generally decreases, thereby resulting in difficulty in solar cell application. Thus, there has been a need for developing a novel electrolyte or developing and applying a novel material, which may replace an electrolyte, to solve the above-described disadvantages.
In general, a ruthenium (Ru) metal complex has been widely used as a dye used for the dye-sensitized solar cell, but the ruthenium metal complex has a disadvantage in that the ruthenium metal complex is highly expensive and is difficult to purify. Further, it takes a long adsorption time from at least 2 hours up to 24 hours for an organic dye including ruthenium metal to be adsorbed onto a semiconductor layer, and thus there is a disadvantage in that the time taken for the manufacturing process is increased, and there is a limitation in that high energy conversion efficiency is achieved only when a thickness of the semiconductor layer is at least about 10 μm. Thus, there have been attempts to use a dye other than the ruthenium metal complex, but even in these cases, energy conversion efficiency of only a maximum of about 3% is achieved when the thickness of the semiconductor layer is around 10 μm, and there is a problem in that high energy conversion efficiency is not achieved when the thickness of the semiconductor layer is thinner than 10 μm.